This invention relates to implantable pacemakers and more particularly to rate-responsive implantable pacemakers that monitor the variation in the respiration or activity of the user.
Early pacemakers comprised a fixed rate stimulation pulse generator that could be reset on demand by sensed a trial and/or ventricular depolarizations. Modem pacemakers include complex stimulation pulse generators, sense amplifiers and leads which can be configured or programmed to operate in single or dual chamber modes of operation, delivering pacing stimuli to the atrium and/or ventricle at fixed rates or rates that vary between an upper rate limit and a lower rate limit. More recently, single and dual chamber pacemakers have been developed that respond to physiologic sensors which, with greater or lesser degrees of specificity, sense the body's demand for oxygenated blood and regulate the pacing rate accordingly.
For example, rate responsive pacing systems have been developed and marketed which rely upon the patient's rate of respiration. Such pacemakers are described, for example, in U.S. Pat. No. 3,593,718 and 4,596,251 and have been commercialized by Biotec and Telectronics. These pacemakers use an impedance pneumograph for acquiring a respiration signal. Such an impedance pneumograph requires additional electrodes and a separate signal generator, in most instances, for impedance measurement. More recently, it has been proposed to employ the variation of the peak-to-peak amplitude of the EGM signals as a rate control signal on the premise that the amplitude varies as a function of the patient's activity and/or respiration as disclosed in U.S. Pat. No. 4,757,815.